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  1. 1. Positioning with BluetoothJosef Hallberg, Marcus Nilsson, K˚are SynnesLule˚a University of Technology / Centre for Distance-spanning TechnologyDepartment of Computer Science and Electrical Engineering971 87 Lule˚a, Sweden.{Josef.Hallberg, Marcus.Nilsson, K˚are Synnes}@cdt.luth.seAbstractThis paper presents an evaluation of Bluetooth positioningin a general positioning platform. Proceeding the evalua-tion a Bluetooth based positioning system was implementedin order to complement the theoretical evaluation with em-pirical tests.Three different ways of positioning with Bluetooth havebeen developed. With a registered positioning service aBluetooth device has an active role in the positioning taskas it sends a position on request. A Bluetooth device canalso take a more passive role in a positioning task, wherethe unique address of the device is used by a connected de-vice to look up respective position in a database. It is alsopossible to forward a position gained from the positioningplatform using the peer to peer characteristics in Bluetooth.This paper does also contain a discussion on the the-oretical time requirements for a positioning system basedon Bluetooth. Empirical tests show that these requirementshold.1 IntroductionLocation awareness create possibilities for new applicationson different devices. Today we see Bluetooth [1] being in-tegrated on a variety of devices. On some of these the onlytechnology available for position tasks is Bluetooth, on oth-ers a cooperation between other technologies and Bluetoothwould give an advantage in position tasks.As many Bluetooth devices as possible should be asso-ciated with a position in order to get good position sourcecoverage. In order to utilize surrounding Bluetooth devicesfor positioning a Bluetooth positioning server needs to bemobile and the position should change automatically as thedevice is moved. With the use of a platform integrating sev-eral positioning system like Alipes [2] this is possible. Anyof the integrated positioning systems can update the positionin the platform, which will be sent to Bluetooth positioningclients using the peer to peer characteristics in Bluetooth.To further improve position source coverage computers us-ing Bluetooth for other tasks than positioning can be used.0-7803-7661-7/03/$17.00 c 2003 IEEEAn office computer using Bluetooth to connect a wirelesskeyboard and mouse can be turned into a position sourcewithout the need of any added software.2 PositioningIn this project, we have developed two different methods forthe positioning task. The direct method, where the Blue-tooth device must be programmed to handle the position-ing task, and the indirect method, where no programming isneeded. If more than one position is collected from eithermethod in a single run, triangulation is done in order to geta more accurate position. The flowchart of the positioningtask can be seen in figure 1With the direct method the device can operate standaloneand do not need any kind of connection to other servicesor devices. The client asks the Bluetooth device if it has apositioning service. If the Bluetooth device answers posi-tively on this request the client will connect to this serviceto retrieve a position. If the answer is negative the clientwill fall back to the indirect method. The indirect methodwas developed to make it possible to use devices that arenot programmable or not capable to incorporate the posi-tioning server. With this method the client will look at theunique address of the communicating device and use thisaddress as a key to lookup the position in a database. Theserver for this database, which we call a location server, canrun locally or on a different computer on the network. Byhaving both these methods we have made it possible to usealmost every Bluetooth device as a positioning source, with-out compromising the mobility aspect or requiring special-ized hardware.Both of these methods can of course be used with dy-namic positions, such as in a mobile unit. The only criteriais that the device must be able to change the position locallyor in the positioning server. However we believe that theBluetooth positioning service is the one that is best suitedfor mobile units. This method does not require any otherconnection than the connection between the position serverand the receiver. The position in a mobile unit can be re-ceived from any other positioning source such as GPS oranother Bluetooth positioning device.
  2. 2. Connect tonearbydeviceReport positionIndirect MethodAsk location serverfor position ofdeviceAsk forpositioningserviceServicefoundNoserviceDirect MethodAsk the device forits positionFigure 1: Flowchart for positioning task2.1 Position a moving targetThe time (t) in seconds that it takes to travel through a circlepassing the center with a diameter (d) at a speed of s is givenby:t = d/sIf we assume that a person has an average walking speedof 1.2 m/s and that the circle around the Bluetooth positionsource where the signal strength is high enough has 20 m indiameter then the time for traveling through this circle wouldbe 16.7 s. This mean that in order to receive a position froma Bluetooth positioning server the whole process must nottake more that 16.7 s to be successful. If we also consider aBluetooth device is not constantly looking for new devicesthen this time will decrease even more. The time betweendevice scans must be at least 1.28 s and will therefore giveus a worst case of 15.4 s for the maximum time for a positionretrieval. This is of course only applicable if we are lookingat only one positioning server. When more position serversare present at the same time we must consider the possibilitythat a new device can become reachable when communica-tion with other devices is ongoing. If this communicationlasts longer then the time between device scans the max-imum time allowed for a successful position retrieval willdecrease even more.3 Theoretical evaluationHow well does Bluetooth work as a source for positions?Let us look at how Bluetooth can work within the importantaspects for radio based positioning in a mobile context.3.1 SecurityIf people will exchange their position with others it is veryimportant that this information is received by the right re-ceiver and that it is not possible to eavesdrop on the infor-mation exchange [3]. Bluetooth has many security features,both authentication and encryption of the link is incorpo-rated.3.2 Physical aspectsThere are a number of aspects to take into considerationwhen it comes to wireless communication. Such aspects canbe regarding range, angle and power consumption. For a po-sitioning task it is also interesting to know what level of ac-curacy that is to expect from the device. For mobile devicesit is also important to conserve power and thus there needsto be a balance between range and power consumption.Bluetooth has a typical range from 10m to 100m depend-ing on the power class of the device. At current date class 3,with a typical 10m radius is the most common.Positioning systems are most common in mobile contextswhere power is limited and it is therefore important to havesystems that conserves power. Bluetooth is designed to con-sume little power and is suitable for mobile devices. Blue-tooth also has different modes for conserving power depend-ing on how the device is used. There is also a possibilityto negotiate the transmission power with another device toreach the lowest possible power consumption with remain-ing connection. However, power consumption is still an is-sue.3.3 AccuracyAccuracy in this Bluetooth positioning system is dependenton the maximum range of the system. The longer the rangethe higher the error can be. This is based on the assumptionthat only one position is read. However, there are methodsof improving the accuracy. Bluetooth for example has pointto multi point connectivity and may be able to read severalpositioning sources at once. The optional received signalstrength function in Bluetooth could possibly be used to cal-culate the distance to the positioning source although signalstrength tend to vary a lot if there are obstacles in the wayor if there is interference. Combining these two techniqueswould give a more accurate position than each of them sep-arately.3.4 AutomationThe user would not want to obtain the position manually,thus the position retrieval needs to be an automatic process.For this automatic device discovery and automatic servicediscovery helps. The automatic service discovery checks ifthe positioning service is present in the discovered device.If there is no such service there is no need to ask for theposition and the positioning client should fall back on other
  3. 3. 05000100001500020000250001 2 3 4 5 6 7 8test numbermsStartuptimeBDECAFigure 2: Results from test-runsmethods for positioning. Bluetooth is designed with bothdevice and service discovery and a position task over Blue-tooth can therefore be made automatic with ease.3.5 ReliabilityFor position acquiring it is important that the link is reliable.Packet loss and bit errors have to be handled in some way tomake the link reliable, as an unreliable link will increase thetime for the position retrieval or even make it fail. Bluetoothworks in the ISM band which is a very crowded frequencyband. Bluetooth tries to avoid this interference by hoppingin frequency 1600 times per second. Bluetooth can detectan error, if it should occur, with Cyclic Redundancy Checkand restore corrupted data with Forward Error Correction.4 Empirical TestsWe have investigated the time for a successful position re-trieval with non moving clients and position sources. Theposition sources are a mix of Bluetooth devices running aposition service and devices whose unique address is presentin the location server locally running at the client. Five de-vices excluding the client device were placed at differentlocations in a corridor, see figure 3. The optional Blue-tooth API for signal strength has very low resolution andis therefore very limited in it’s usefulness for calculation ofdistance. Some simple test confirmed that the value was un-reliable and no relation between signal strength and distancecould be found. Because of this the accuracy was hard-coded to the maximum range of 10 meters.The test was performed by letting the client search for aposition in its environment and the time was recorded forevery connection. The resulting position of the triangulationand the total time for the search was reported in the end.Figure 2 shows the result of our test where every time forthe test run is a represented by a bar that is divided into sec-tions for the separated devices. We made 8 tests by handand recorded the time for every connection per test. Thestartup time in figure 2 is the difference between the sum ofthe times for every connection and the measured time for theDevice A B C D EFailed connections 2 1 6 1 1Table 1: number of failed connections for every deviceFigure 3: The field-test setupwhole test. We can see that this time is almost the same forevery test at an average of 5.3 s. Most of this time is spentsearching for nearby devices, and it was set to search for 5 s.The connection to one device will never exceed 5.2 s as theconnection will time out and the client will try to connect tothe next device instead.The amount of failed connections to a device can be seenin table 1. The average position error is 1.7m +/- 1.7m basedon these few tests. For a more accurate error estimate weneed more test results. The lack of a satisfying test envi-ronment for a longer period and the fact that we have somepossible major performance gains in development for exam-ple parallel connections, made us wait with a more completetest. We decided that the results we have will give us suffi-cient accuracy at this time.4.1 Performance evaluationThe startup time is static and does not change dependent onhow many devices we are connecting to. The rest of thetime is dependent on the amount of connected devices andthe time that every connection takes, see figure 2. We cancalculate that the average time in seconds for a position re-trieval in this test was 19.2 s but the worst case will be5.3 + 5.2 ∗ n
  4. 4. Where n is the amount of devices that are in range. In ourtest with five devices this would be calculated to 31.3 sAs seen in figure 3, device C is approximately 10 meterfrom the client. The Bluetooth devices used in this test are ofpower-class 3 which have a typical max range of 10 meters.Our test confirms this as we see in table 1 that device C havevery few successful connections. Our hard-coded value forthe distance (10 meters) was therefore a good choice. AsBluetooth is a radio based communication there are severalthings that can disturb the connection between devices. Wecould see that Bluetooth can be improved in dealing withthis as we never did manage to connect to all devices overone test run. Triangulation will in most cases give a betterresult than 10 meters. Even though all our tests give us asmaller error than 10 meters the theoretical worst case isstill 10 meters.5 Related workPositioning systems have been an area of research for along time and many different methods and technologies havebeen used. One of the well known system using techniquesfor location awareness is the ParcTab [4]. The ParcTab is arather cheap technology. It uses infrared light for networkconnection and position information. The downside withthe ParcTab system is that it uses specialized hardware forthe positioning system and can only give a persons locationby which room he or she is in. Another system that havebeen developed is ORL [5] which is using ultrasonic tech-nology to determine a persons location. This system havea quite good position calculation but suffer from requiringthe user to use specialized hardware to retrieve a position.There are also systems like RADAR [6] which like our sys-tem uses technology that already is available in mobile units.RADAR share many of the ideas and requirements with usbut does not consider that a mobile user can move and theidea of moving position sources. There is also work beingdone on an official positioning system standard for Blue-tooth in the Bluetooth SIG. This standard is under devel-opment but is not public. We have not been able to fullyunderstand what differences there are from our work. Thelittle information we have managed to see shows that thisstandard will require the device to be manufactured for po-sitioning services. If this is the case our implementation canbe an interesting extension to the official standard.6 Discussion and future workThe test of our implementation showed that Bluetooth canfunction as a position transmitter and/or receiver. We also sethat our implementation will successfully receive a positionif only one position source is present. The worst case forthe position retrieval from one source is 10.5 s (5.3 s forthe device discovery plus overhead and 5.2 s for connectionto the device). In section 2.1 we concluded that with oneposition source the maximum time for position retrieval is15.4 s. 10.5 s is well under 15.4 s which proves that ourimplementation will function satisfactory in this scenario.When more than one position source is present as in ourtest in section 4 the time for the worst case scenario will ex-ceed 15.4 even after only 2 devices. In this scenario we mayget a position dependent on in which order we retrieve thepositions and how the different devices are located aroundthe client. This is a result of the fact that the current imple-mentation is not optimized for more than one device and weare expecting great improvements in this area.One thing that we want to change is that the current im-plementation only connects to one device at a time. Mostof the connection time is spent waiting on the other deviceand if a parallel connection to all devices could be made thiswould decrease the total time to the startup time plus thetime for the longest connection. For example, test 1 in fig-ure 2 would only take around 10.4 ms and in that case wewould have enough time for positioning when walking aswell.Another interesting improvement would be to let the de-vice maintain connection to the server as long as it is ableto. If a device wants to check for a position over and overit would be spared the effort of reconnecting to the knownserver. This would most likely save both time and power forthe device. This however does not only come with advan-tages but also flaws. There is a limit of seven connectionsthat a server may uphold and if devices were to stay con-nected it would run out of free connections. This would bothhinder new devices from getting a position but it would alsohinder the server from doing anything else that it is usingthe Bluetooth for. A few restrictions could be added in orderto limit the downsides of letting devices stay connected to aserver. One of these restrictions could be to limit the num-ber of connections allowed for the positioning service. TheServer could then use ”first in, first out” (FIFO) to supplyconnections to new devices that wishes to connect.There would also be a gain in developing a cache systemwhere recently retrieved position that are static can be readlocally instead of spending time connecting to that particulardevice over and over again.We must also conduct more tests that uses moving posi-tion sources and clients to determine the performance of oursystem in that scenario.7 ConclusionsIn this paper we have presented a positioning system forBluetooth enabled devices. Bluetooth is an inexpensive andsmall size solution and will probably be integrated in a rangeof different devices. With a positioning system based onBluetooth we are able to make these devices location aware.This will open up for new applications and services that canbe developed on these devices.We have showed that the current implementation is fastenough for the task at least for one device. We have also
  5. 5. showed that future changes will probably make it even fasterand by that make it suitable for more devices and in thathigher accuracy.With the use of our location server, even the devices thatcannot run any third party software can be used as positionsources.8 AcknowlegementsThis work was done in the Alipes project and special thanksgoes to our sponsors CDT, MRC and SITI. Thanks to JamesNord for his work with the Alipes platform.References[1] Jennifer Bray and Charles F Sturman. Bluetooth: Con-nect Without Cables. Prentice-Hall, Upper Saddle River,2001.[2] James Nord, K˚are Synnes, and Peter Parnes. An archi-tecture for location aware applications. In HICSS-35.Hawaii’s International Conference on System Sciences,January 2002,.[3] K˚are Synnes, James Nord, and Peter Parnes. Locationprivacy in the alipes platform. Technical report, Lule˚aUniversity of Technology/Department of Computer Sci-ence and Electric Engineering, 2002.[4] Bill N. Schilit, Norman Adams, Rich Gold, Michael M.Tso, and Roy Want. The PARCTAB mobile comput-ing system. In Workshop on Workstation Operating Sys-tems, pages 34–39, 1993.[5] A. Ward, A. Jones, and A. Hopper. A new location tech-nique for the active office, 1997.[6] V. Bahl and V. N. Padmanabhan. Radar: An in-buildingrf-based user location and tracking system. In Infocom2002, Tel-Aviv, Israel, 2000. IEEE Infocom.